1. Field of the Invention
The present invention relates to an optical apparatus such as a broadcasting lens or a video camera lens, and more particularly, to a focus detecting apparatus for automatic focus function, a lens apparatus including the same, an image pickup apparatus, and a method of detecting a defocus amount.
2. Description of the Related Art
Conventionally, there are various proposals as an automatic focus (AF) technology for an image pickup apparatus such as a camera or a video camera. For instance, the following automatic focus adjustment method is well known. Light beams from an object passing through different exit pupil regions of an image pickup lens are guided to form images on a pair of line sensors, and the object images are photoelectrically converted so as to obtain a pair of image signals. Then, a relative position displacement amount between the image signals is determined. Based on this displacement amount, a defocus amount of the object is calculated so as to drive the image pickup lens for the automatic focus adjustment.
This AF system using phase difference detection can determine a focusing position of a focus lens from an object distance, and hence has a feature that focusing can be performed faster than in a contrast AF system.
Japanese Patent Application Laid-Open No. 2010-66712 discloses a method of increasing a defocus range of focus detection by decreasing the number of pixels to be subject to correlation calculation on the two line sensors to be used for normal phase difference detection so as to increase a pixel shift amount for the correlation calculation, in order to reduce the probability of automatically entering a scan AF mode for detecting a focus shift while driving the focus lens when the focus detection by the phase difference detection method cannot be performed.
However, in Japanese Patent Application Laid-Open No. 2010-66712, the defocus range for focus detection can be widened, but detection accuracy is lowered because the number of pixels to be subject to correlation calculation is reduced. In addition, false detection is apt to occur depending on an object position on an AF sensor.
In a phase difference AF method as a detection method using a light beam different from that of an image pickup system, it is necessary to adjust an imaging position (sensor back) of a phase difference AF sensor as an AF detection unit with respect to an image of the image pickup system. This is due to an error between an optical distance from the separating optical system to the phase difference AF sensor and an optical distance from the separating optical system to the image plane, and a mounting accuracy of the phase difference AF sensor. However, when using a line sensor or the like capable of detecting a phase difference in multiple positions in a photographed image, because a sensor back amount is different for each line sensor, the sensor back amount as an adjustment amount for each line sensor is stored, and a focusing operation is performed based on the stored sensor back amount. In other words, as to one object image, on a pair of line sensors, it is necessary to perform the focusing operation by adjusting, for each pair of line sensors, a state where the images are formed at positions displaced in a longitudinal direction of the line sensor with respect to an ideal state where the images are formed at the same position in the longitudinal direction of the line sensor.
However, due to a displacement amount of this sensor back, if an object as an AF target exists near a sensor end, the same object cannot be a target of the comparison between the pair of line sensors, and thus false detection may occur.